The CDMA (Code Division Multiple Access) communication system is a communication system that allows communication for a plurality of users by assigning a discrete spreading code to each user. The CDMA communication system is characterized by making positive use of multipaths.
In wireless communication, a signal transmitted from a transmitter is affected by fading that occurs in connection with the movement of the transmitter or a receiver, diffraction due to a shield, attenuation, or the like. Thus, the signal transmitted from the transmitter is received by the receiver as multiple signals. The received multiple signals are referred to as “multipaths”. Each individual reception signal in multipaths is referred to as a “path”.
In TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access), when one signal (a main signal) of such multipaths is to be received, other signals are regarded as noise that disturbs the main signal.
In CDMA, however, the multipaths are separated into individual paths and all the paths are processed as main signals. Such processing is performed to enhance the reception sensitivity. Rake receivers have a search function that involves separating the paths, determining individual reception timings, and assigning them to fingers, A finger is meant to have a receiving function corresponding to one reception timing.
FIG. 1 is a diagram showing the basic configuration of a conventional rake receiver for achieving the search function described above. As shown in FIG. 1, the rake receiver has a correlation-value calculation section 2, a delayed-profile creation section 3, a path selection section 4, and a finger section 5.
The correlation-value calculation section 2 calculates correlation values between spreading codes and reception signals input from an input terminal 1, at a certain cycle at all timings at which rake reception is possible. The correlation-value calculation section 2 issues the calculated correlation values to the delayed-profile creation section 3. The “correlation value” is a value determined by the following equation (1).
R[t][i]: an ith chip input signal at reception timing t, C[i]: an ith chip spreading code
N: The number of chips for determining correlation
Cor[f][n]: a correlation value of the nth determination at reception timing=t
                                          Cor            ⁡                          [              t              ]                                ⁢                                          [          n          ]                =                              ∑                          i              =              0                                      N              -              1                                ⁢                                    (                                                                    R                    ⁡                                          [                      t                      ]                                                        ⁢                                                                          [                  i                  ]                                ×                                  C                  ⁡                                      [                    i                    ]                                                              )                        ×                          (                                                                    R                    ⁡                                          [                      t                      ]                                                        ⁢                                                                          [                  i                  ]                                ×                                  C                  ⁡                                      [                    i                    ]                                                              )                        *                                              (        1        )            
where A* represents the complex conjugate of A.
In the following description, the total number of reception timings will be expressed by “Ntmg”.
The delayed-profile creation section 3 obtains correlation values for each reception timing, and determines an average correlation value by adding the correlation values by a predetermined number of times, to thereby create a delayed profile. FIG. 2 shows a delayed profile for individual reception timings.
The path selection section 4 compares the magnitudes of the average correlation values in the delayed profile, selects reception timing to be assigned to each finger, and issues the reception timing to the finger section 5.
In general, when a finger performs reception at reception timing at which the average correlation value is large, it is expected that a signal having a high Eb/IO (the ratio of energy per bit to interference power density) can be received. Here, FIG. 3 is a diagram showing relationships between reception timings shown in FIG. 2 and propagation channels. Reception timings A and A′ have the same propagation channel from a transmitter to a receiver. In contrast, the reception timing B has a propagation channel from the transmitter to the receiver via a reflector. Thus, the propagation channel of the reception timing B is different from the propagation channel of the reception timings A and A′.
Meanwhile, when the reception timing A′ and the reception timing B, which are shown in FIG. 2, are compared with each other, the average correlation value at the reception timing A′ is larger. Thus, it appears to be better to assign the reception timing A′ to a finger. When fading is considered, however, if the reception timing A has already been assigned to a finger, it is expected that assigning the reception timing B, which is different from the reception timing A in propagation channel from the transmitter to the receiver, to a finger can rather improve the characteristics, even though the average correlation value at the reception timing B is smaller.
Since the reception timings A and A′ have the same propagation channel, they are affected by the same fading. Thus, for example, when the reception timing A is affected by fading and thus the reception power decreases, the reception power at the reception timing A′ also decreases. Consequently, when only the reception timings A and A′ are assigned to fingers, the reception characteristics can deteriorate.
In contrast, even when the reception power at the reception timing A decreases due to fading, this decrease does not necessarily mean a decrease at the reception timing B, since the reception timings A and B have the different propagation channels. This can therefore prevent deterioration of the reception characteristics which is due to fading. This is basically because, when the propagation channels are different, the influences of fading are also different.
The number of fingers is generally limited due to restrictions of hardware. Thus, assignment of paths of different propagation channels to fingers with efficiency has a great influence on the reception characteristics in rake reception.
The number of fingers which is limited due to restrictions of hardware will hereinafter be expressed by “Nfgr”.
Conventionally, in path assignment to fingers, when comparison of reception timings shows that the difference (A=abs (reception timing for the path A)−(reception timing for the path B)) is a predetermined threshold or smaller (A≦TH), the paths are recognized as having the same propagation channel and thus only one of the paths is assigned to a finger. On the other hand, when the difference is greater than the predetermined threshold (A>TH), the paths A and B are recognized as having channels different from each other.
In this case, abs (x) represents the absolute value of x and TH represents a predetermined threshold.
In such a conventional rake receiver, during finger assignment, whether or not paths have the same channel is determined based on only the difference between reception timings. Thus, paths which are almost the same in reception timing but are different in propagation channels are determined as being an identical path and one of the paths is deleted. Conversely, in some case, regardless of being an identical propagation channel, the path is determined as having different propagation channels for assignment to fingers. Thus, paths cannot be efficiently assigned,
That is, in the conventional rake receiver, for paths which are different in propagation channels but are almost the same in propagation distance, a determination is made as being the same propagation channel. Thus, reception characteristics can deteriorate during fading.
An object of the present invention is to provide a rake receiver and a receiving method for efficiently assigning paths to fingers in such a manner that path separation is performed by looking at correlation between shift states of magnitudes of correlation values.